Various catalysts have so far been developed as catalysts for purification of exhaust gas with which noxious substances emitted from an internal combustion engine of an automobile or the like are treated for purification. For example, as a three-way catalyst which oxidizes CO and HC in exhaust gas and simultaneously reduces NOx therein for purification, a catalyst has widely been known in which a noble metal such as platinum or rhodium is supported on a catalyst support made of alumina, silica, zirconia, titania, and the like.
Meanwhile, as a method for producing such a catalyst support made of a composite metal oxide, the so-called coprecipitation method or the so-called hydrothermal synthesis method is generally employed. Specifically, in the coprecipitation method, an alkali is added to a solution containing multiple metal ion species, and thereby a supersaturation state where the ion concentration product in the solution is higher than the solubility product is achieved. As a result, insoluble salts of multiple kinds of metals precipitate and deposit simultaneously. Meanwhile, the hydrothermal synthesis method is a synthesis method conducted in the presence of hot water at high temperature and high pressure.
For example, International Application Japanese-Phase Publication No. 2008-526660 (Patent Literature 1) discloses a method for the formation of a composite hydroxide or oxide as a method for forming a composite hydroxide or oxide containing alumina and zirconia, and optionally containing at least one kind selected from ceria, lanthana, praseodymia, samaria, yttria, and other rare earth oxides. This method for the formation of a composite hydroxide or oxide comprising the steps of: preparing an aqueous metal salt solution consisting of water-soluble salts of metals corresponding to metals in the aforementioned composite oxide; and forming a hydroxide precipitate slurry by combining the aqueous metal salt solution with an aqueous solution of a caustic alkali in a reaction vessel with agitation at a pH greater than 8.5 to precipitate out all the metal species.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2002-211908 (Patent Literature 2), which was applied by the same applicants as those of the present application, discloses a composite oxide powder produced by a method comprising: from an aqueous solution or a water-containing solution in which a cerium compound and a compound of at least one metal selected from Al, Ti and Si are dissolved, allowing the formation of a precipitate of a precursor of cerium oxide and a precursor of an oxide of the at least one metal selected from Al, Ti and Si or a compound of these precursors; aging the precipitate in a suspension state where the aqueous solution or the water-containing solution serves as a dispersant or in a state where water is sufficiently present in a system; and then calcining the precipitate. Patent Literature 2 states that, in such a composite oxide powder, ceria, and the oxide of the at least one metal selected from alumina, titania, and silica are dispersed on the no scale, and that when microanalysis on a non-overlapped single particle is carried out by means of EDS using a FE-STEM with a beam of 5-nm diameter, Ce and the at least one metal selected from Al, Ti, and Si are detected at composition ratios in ±20% of their respective feed composition ratios at 90% or more of analytical points.
However, the composite metal oxide is obtained by the so-called coprecipitation method in the invention described in each of these Patent Literatures. Such a composite metal oxide prepared by the conventional coprecipitation method or the conventional hydrothermal synthesis method has a limitation on improvement in the uniformly of the metal oxide, and a limitation on improvement in heat resistance. Accordingly, a catalyst obtained by using such a composite metal oxide as a support is not necessarily satisfactory because of its limitations on improvement in the catalytic activities and the durability thereof at high temperature.